The present invention relates to improvements in temperature-compensated microwave oscillators of the type used, for example, in Doppler-based motion-detection systems.
It is well known that the frequency of oscillation of a microwave oscillator is temperature-dependent. Left uncompensated for ambient temperature changes, the output frequency of a conventional X-band Gunn oscillator can vary by as much as 800 kHz.per degree-Centrigrade. This variation is primarily the result of metal expansion of the oscillator's metal housing which defines the resonant cavity. As the ambient temperature increases, the cavity expands in size, causing a corresponding decrease in the cavity's resonant frequency.
Due to FCC requirements governing acceptable frequency bandpasses within which certain types of microwave oscillators can operate, it is common for manufacturers of such oscillators to incorporate some means for compensating for ambient temperature changes so that the output frequency remains substantially stable with temperature. A common technique is one which exploits a differential metal expansion between the oscillator's metal housing and a so-called "tuning rod" which extends into the cavity as a stub. In general, the further the tuning rod extends into the cavity, the lower the frequency of oscillation. The rod is mounted at one end of a tube extending outwardly from the oscillator housing. Thus, as the temperature increases, the cavity becomes larger and the resonant frequency drops. But at the same time, the tube grows in length, thereby withdrawing the tuning rod from the cavity and causing the resonant frequency to increase. In theory, by properly selecting the materials of the housing and the tuning rod, the frequency drop caused by the growth of the cavity will be offset by the frequency increase caused by the partial extraction of the tuning rod from the cavity, and the oscillator frequency will remain constant with temperature. While it is never possible to entirely eliminate frequency drift, it is possible to reduce the frequency drift to less than 50 kHz per degree Centigrade.
In the differential expansion scheme described above, it is common to select a tuning rod material which exhibits very little expansion with temperature. Otherwise, the length of the tube in which the tuning rod is mounted need be relatively long in order to achieve the desired movement (as a result of temperature change) of the free end of the rod in the cavity. A preferred material is alumina. The use of such materials can add significantly to the product cost and, desirably, should be avoided.